Low voc laminating formulations

ABSTRACT

The invention is a formulation that contains: (1) a low-viscosity epoxy resin; (2) a phenolic chain extender whose concentration is less that 0.6 equivalents phenolic hydroxyl group per equivalent of the low-viscosity epoxy resin; (3) a catalyst that promotes self-curing reactions between epoxy groups; (4) an inhibitor that inhibits the activity of the catalyst under “B-staging” conditions; (5) less than 25 weight percent of a volatile organic solvent; and (6) optionally, a multifunctional cross-linking agent. The formulation contains low levels of volatile organic solvent, and can be used to make electrical laminates. It builds molecular weight controllably in B-staging, so that dripping is avoided but the prepreg can be easily laminated.

[0001] The present invention relates to the art of curableepoxy-resin-containing formulations, and particularly to formulationsuseful for making electrical laminates.

[0002] It is known to make electrical laminates and other compositesfrom a fibrous substrate and an epoxy-containing matrix resin. Examplesof suitable processes usually contain the following steps:

[0003] (1) an epoxy-containing formulation is applied to a substrate byrolling, dipping, spraying, other known techniques and/or combinationsthereof. The substrate is typically a woven or nonwoven fiber matcontaining, for instance, glass fibers.

[0004] (2) The impregnated substrate is “B-staged” by heating at atemperature sufficient to draw off solvent in the epoxy formulation andoptionally to partially cure the epoxy formulation, so that theimpregnated substrate can be handled easily. The “B-staging” step isusually carried out at a temperature between 90° C. and 210° C. and fora time between 1 minute and 15 minutes. The impregnated substrate thatresults from B-staging is called a prepreg. The temperature is mostcommonly 100° C. for composites and 130° C. to 180° C. for electricallaminates.

[0005] (3) One or more sheets of prepreg are stacked in alternatinglayers with one or more sheets of a conductive material, such as copperfoil, if an electrical laminate is desired.

[0006] (4) The laid-up sheets are pressed at high temperature andpressure for a time sufficient to cure the resin and form a laminate.The temperature of lamination is usually between 100° C. and 230° C.,and is most often between 165° C. and 190° C. The lamination step mayalso be carried out in two or more stages, such as a first stage between100° C. and 150° C. and a second stage at between 165° C. and 190° C.The pressure is usually between 50 N/cm² and 500 N/cm². The laminationstep is usually carried on for 10 to 100 minutes, and most often for 45to 90 minutes. The lamination step may optionally be carried out athigher temperatures for shorter times (such as in continuous laminationprocesses) or for longer times at lower temperatures (such as in lowenergy press processes).

[0007] (5) Optionally, the resulting copper-clad laminate may bepost-treated by heating for a time at high temperature and ambientpressure. The temperatures of post-treatment are usually between 120° C.and 250° C. The post-treatment time usually is between 30 minutes and 12hours.

[0008] The electrical laminates and processes by which they are made aredescribed in greater detail in numerous references, such as U.S. Pat.No. 5,314,720 (May 24, 1994) and Delmonte, Hoggatt & May;“Fiber-reinforced Epoxy Composites,” Epoxy Resins, Chemistry andTechnology (2d Ed.) at 889-921 (Marcel Dekker, Inc. 1988).

[0009] The formulations that are used in such processes typicallycontain: (1) an advanced epoxy resin having an EEW of 400 to 530; (2) acuring agent such as dicyandiamide; (3) a catalyst to promote thereaction of the resin and the curing agent, such as 2-methylimidazole;and (4) 30 to 40 weight percent of a volatile organic solvent such as aketone, a glycol ether, dimethylformamide, or xylenes. It is also knownthat the formulation may contain certain other additives. For instance:

[0010] (1) Formulations that contain boric acid are taught in U.S. Pat.No. 5,308,895 (May 3, 1994) and U.S. Pat. No. 5,314,720 (May 24,1994).

[0011] (2) Formulations that contain a chain extender such as bisphenolA or tetrabromo-bisphenol A are described in references such as theprevious two U.S. patents and in European Patent Application 92101367.8(published Jan. 28, 1992) and U.S. Pat. No. 3,738,862 (Jun. 12, 1973).

[0012] It would be desirable to reduce the quantities of volatileorganic solvent that are used in the formulation. Volatile organicsolvents are costly and must be cleaned out of the effluent gas from theB-stage before that gas is returned to the atmosphere. However, the lowVOC (volatile organic compound) formulation must be selected to providea suitable viscosity, since viscosity is critical in laminate makingprocesses. See, for example, Delmonte, Hoggatt & May at 903. The levelof VOC in ordinary formulations can not be reduced because the viscosityof the formulation would be too high. High viscosity resins distort theposition of fibers in the substrate, and are difficult to impregnateinto the substrate.

[0013] The viscosity of low VOC formulations may be reduced if theadvanced epoxy resin is replaced with a lower-molecular-weight resin anda chain extender. Such low-viscosity formulations have been reported forother uses. For instance, EPO Publication 0 260 768 A2 (Mar. 23, 1988)teaches an encapsulation formulation which contains liquid epoxy resin,0.6 to 1 equivalent of bisphenol, and an amine-(boron trifluoride)catalyst. Formulations which contain liquid epoxy resin, chain extenderand catalysts or catalyst complexes have been partially-cured, and thepartially-cured resins are used in curable resins in PCT PublicationWO86/00627 (Jan. 30, 1986).

[0014] Formulations which contain liquid epoxy resin and chain extenderhave not commonly been used in laminating processes, because theirviscosity in the treater and prepreg is often too low. The formulationsrun and drip in the treater before the B-stage is complete. Furthermore,the formulations flow too much after the prepreg is put into thelaminating press. The resin is forced out of the laminate and into thepress, and the resulting laminate is too thin.

[0015] Extra catalysts may be added to the formulation to encouragequick reaction of epoxy resin and chain extender in the treater, so thathigher molecular weight advanced resins are produced before drippingoccurs. However, those catalysts also accelerate curing of the resinwith the curing agent. It is difficult to prevent the viscosity frombuilding too high for effective lamination. Moreover, formulations whichcontain too much catalyst have a short shelf- or pot-life, and theresulting prepregs have a short shelf-life.

[0016] What is needed is a low VOC formulation that: (1) has alow-viscosity in the impregnation step; (2) builds molecular weight byadvancement rapidly and controllably in the B-stage in order to minimizedripping; (3) controls undesirable curing reactions, to preventexcessive growth of molecular weight in the treater or during storage;and (4) provides a B-staged prepreg with sufficient viscosity to belaminated without significant loss of resin. It is a further object ofthe invention to present an epoxy resin formulation which is compatiblewith existing manufacturing equipment for electrical laminates preparedfrom advanced epoxy resin formulations.

[0017] Applicants invention provides these features and others as willbecome apparent from the entire description and examples herein.

[0018] One aspect of the present invention is a formulation comprising:

[0019] (1) a low-viscosity epoxy resin;

[0020] (2) a phenolic chain extender whose concentration is less than0.6 equivalents of phenolic hydroxyl group per equivalent of thelow-viscosity epoxy resin;

[0021] (3) a catalyst that promotes self-curing reactions between epoxygroups;

[0022] (4) an inhibitor which is a Lewis acid;

[0023] (5) less than 25 weight percent of a volatile organic solvent;and

[0024] (6) optionally, a multifunctional cross-linking agent.

[0025] A second aspect of the invention is a preliminary formulationcontaining:

[0026] (1) a low-viscosity epoxy resin;

[0027] (2) a phenolic chain extender whose concentration is less than0.6 equivalents of phenolic hydroxyl group per equivalent of thelow-viscosity epoxy resin;

[0028] (3) a Lewis acid; and

[0029] (4) no more than 20 weight percent of a volatile organic solvent.

[0030] A third aspect of the present invention is the use of aformulation as previously described in the processes to make compositesand electrical laminates that are previously described.

[0031] The composition in the second aspect of the invention is usefulto make formulations in the first aspect of the invention. Thoseformulations are useful for making electrical laminates as previouslydescribed. The compositions may also be used in encapsulation, coatingand structural composite applications. The invention is described morespecifically as follows.

[0032] Formulations of the present invention contain a low-viscosityepoxy resin. The low-viscosity epoxy resin preferably either:

[0033] (1) is liquid at 20° C.; or

[0034] (2) has an average formula weight per epoxy equivalent of no morethan 350 for all non-halogen atoms in the molecule. (For instance,diglycidyl ether of bisphenol A contains no halogen and has an averageformula weight of 340. Therefore, the formula weight per epoxyequivalent of diglycidyl ether of bisphenol A is 170. The diglycidylether of tetrabromobisphenol A has a molecular weight of 656. However,the average formula weight of the non-halogen atoms is 336, and so theaverage formula weight per epoxy equivalent is 168 for non-halogen atomsin diglycidyl ether of tetrabromobisphenol A. Both diglycidyl ether ofbisphenol A and diglycidyl ether of tetrabromobisphenol A are“low-viscosity epoxy resins” within the meaning of this definition.)

[0035] The average formula weight per epoxide equivalent is preferablyno more than 250 and more preferably no more than 190 for non-halogenatoms in the low-viscosity epoxy resin. It is preferably at least 70,more preferably at least 110, and most preferably at least 160 fornon-halogen atoms in the low-viscosity epoxy resin.

[0036] The low-viscosity epoxy resin contains on average more than 1epoxy group per molecule and preferably contains on average at least 1.8epoxy groups per molecule. It preferably contains on average less than 5epoxy groups per molecule, more preferably less than 3 epoxy groups permolecule, and most preferably no more than 2.1 epoxy groups permolecule. (For certain special applications, such as high-temperatureuses, resins with more epoxy groups may be optimum, such astriglycidylether of tris-(hydroxyphenyl)methane, which is commerciallyavailable as TACTIX* 742 resin. (*Trademark of The Dow ChemicalCompany)). The low-viscosity epoxy resin is preferably a glycidyl ether,ester or amide compound.

[0037] The low-viscosity epoxy resin is more preferably the diglycidylether of a diol. The diol preferably contains 2 phenolic hydroxyl groupsper molecule (a dihydric phenol). The diol and the diglycidyl ether arepreferably represented by Formula l:

[0038] wherein:

[0039] each “A” independently represents an aliphatic group, an aromaticgroup or a plurality of aromatic groups (including halogenated and/orsubstituted aromatic groups) linked by a bond or a divalent moiety suchas a lower (C₁-C₆) alkyl group, a carbonyl group, a sulfonyl group or anoxygen atom. Preferably, less than 50 percent of “A” are aliphaticgroups, more preferably less than 30 percent are aliphatic groups, andmost preferably 0 percent are aliphatic groups. The aliphatic groups arepreferably alkyl groups or poly(alkylene oxide) groups. Each “A” is mostpreferably a benzene ring or two benzene rings linked by a lower alkylgroup, or a halogenated variation thereof.

[0040] Each “Q” is a hydroxyl group in the diol, and is a glycidyl ethermoiety represented by Formula II:

[0041] in the epoxy resin.

[0042] Each “R” represents a hydrogen atom, a halogen or a lower alkylgroup. Each “R” is preferably a hydrogen atom.

[0043] “n” represents a number of repeating units. “n” may be on average0 to 2, but it is preferably 0.1 to 0.2 in low-viscosity epoxy resin.

[0044] Examples of preferred diols include resorcinol, catechol,hydroquinone, bisphenol, bisphenol A, bisphenol AP(1,1-bis(4-hydroxylphenyl)-1-phenylethane), bisphenol F, bisphenol K orhalogenated variations thereof. The low-viscosity epoxy resin is mostpreferably a diglycidyl ether of bisphenol A or a diglycidyl ether of ahalogenated bisphenol A. Other useful low-viscosity epoxy resins areglycidyl ether derivatives of 1,1,1-tris-(hydroxyphenyl)-alkanes andhalogenated variations thereof. Examples of suitable epoxy resins andprocesses to make them are also described in H. Lee & K. Neville,Handbook of Epoxy Resins at 2-1 to 3-20 (McGraw-Hill Book Co. 1967).

[0045] The formulations also contain a phenolic chain extender. Thephenolic chain extender may be any compound that contains on averagemore than 1 and less than 3 phenolic hydroxyl groups per molecule. Itpreferably contains on average 1.8 to 2.1 phenolic hydroxyl groups andmore preferably contains about 2 phenolic hydroxyl groups per molecule.The phenolic chain extender has the same broad description and preferredembodiments as the dihydric phenol described previously as the basis forlow-viscosity epoxy resin (except that “n” in Formula I is preferablyless than 0.2, more preferably less than 0.1 and most preferably 0).

[0046] The phenolic chain extender is preferably a liquid or a solidthat is soluble in liquid epoxy resin, in order to minimize the need fora volatile organic solvent. It more preferably has a melting point whichis higher than 100° C., and most preferably has a melting point of atleast about 125° C. and no more than about 300° C. When the phenolicchain extender is non-halogenated, its molecular weight is preferably atleast 110 and more preferably at least 185. The molecular weight ispreferably no more than 800, more preferably no more than 500, and mostpreferably no more than 250. For halogenated phenolic chain extenders,the formula weight of non-halogen atoms in the chain extender preferablymeets the foregoing preferred limitations, and the total molecularweight is preferably within the preferred embodiments plus the formulaweight of the halogen. The phenolic chain extender may optionally be aphenol-capped oligomer (such as an oligomer which meets Formula Iwherein “Q” is a hydroxyl group and “n” is on average between 0.2 and2), but preferably the chain extender is simply a monomer (wherein “n”meets the limits previously described). The phenolic chain extender ismost preferably bisphenol A or brominated bisphenol A.

[0047] The quantity of chain extender should be less than stoichiometricwith the epoxy resin. The chain extender preferably contains no morethan 0.55 hydroxyl equivalents per epoxy equivalent, and more preferablyno more than 0.5 hydroxyl equivalents per epoxy equivalent. The chainextender preferably contains at least 0.1 phenolic hydroxyl equivalentsper epoxy equivalent, and more preferably contains at least 0.2 phenolichydroxy equivalents per epoxy equivalent and most preferably at least0.3 phenolic hydroxy equivalents per epoxy equivalent. When thelow-viscosity epoxy resin is the diglycidyl ether of bisphenol A and thechain extender is tetrabromobisphenol A, the concentration of chainextender is preferably sufficient to provide a resin containing 17 to 30weight percent bromine, and more preferably sufficient to provide aresin containing 19 to 22 weight percent bromine.

[0048] Compositions of the present invention contain a catalyst that cancatalyze epoxy-epoxy curing reactions—the reaction of epoxy groups witheach other to form a cured resin. Such curing reactions are described inVol. 6, Encyclopedia of Poly. Sci. & Eng. (2d Ed.), “Epoxy Resins” at341-343 (J. Wiley & Sons 1986). Examples of suitable catalysts includecompounds containing amine, phosphine, heterocyclic nitrogen, ammonium,phosphonium, arsonium or sulfonium moieties. More preferred catalystsare the heterocyclic nitrogen and amine-containing compounds and evenmore preferred catalysts are heterocyclic nitrogen-containing compounds.

[0049] Catalysts (as distinguished from cross-linkers) preferablycontain on average no more than about 1 active hydrogen moiety permolecule. Active hydrogen moieties include hydrogen atoms bonded to anamine group, a phenolic hydroxyl group, or a carboxylic acid group. Forinstance, the amine and phosphine moieties in catalysts are preferablytertiary amine or phosphine moieties; and the ammonium and phosphoniummoieties are preferably quaternary ammonium and phosphonium moieties.

[0050] Examples of suitable heterocyclic nitrogen catalysts includethose described in Bertram, U.S. Pat. No. 4,925,901 (May 15, 1990).Preferable heterocyclic secondary and tertiary amines ornitrogen-containing catalysts which can be employed herein include, forexample, imidazoles, benzimidazoles, imidazolidines, imidazolines,oxazoles, pyrroles, thiazoles, pyridines, pyrazines, morpholines,pyridazines, pyrimidines, pyrrolidines, pyrazoles, quinoxalines,quinazolines, phthalozines, quinolines, purines, indazoles, indoles,indolazines, phenazines, phenarsazines, phenothiazines, pyrrolines,indolines, piperidines, piperazines and combinations thereof, Especiallypreferred are the alkyl-substituted imidazoles; 2,5-chloro-4-ethylimidazole; and phenyl-substituted imidazoles, and mixtures thereof. Evenmore preferred are N-methylimidazole; 2-methylimidazole;2-ethyl-4-methylimidazole; 1,2-dimethylimidazole; and 2-phenylimidazole.Especially preferred is 2-methylimidazole.

[0051] Among preferred tertiary amines that may be used as catalysts arethose mono- or polyamines having an open-chain or cyclic structure whichhave all of the amine hydrogen replaced by suitable substituents, suchas hydrocarbon radicals, and preferably aliphatic, cycloaliphatic oraromatic radicals. Examples of these amines include, among others,methyl diethanol amine, triethylamine, tributylamine, dimethylbenzylamine, triphenylamine, tricyclohexyl amine, pyridine andquinoline. Preferred amines are the trialkyl, tricycloalkyl and triarylamines, such as triethylamine, triphenylamine,tri-(2,3-dimethylcyclohexyl)amine, and the alkyl dialkanol amines, suchas methyl diethanol amines and the trialkanolamines such astriethanolamine. Weak tertiary amines, for example, amines that inaqueous solutions give a pH less than 10 in aqueous solutions of 1 Mconcentration, are particularly preferred. Especially preferred tertiaryamine catalysts are benzyldimethylamine and tris-dimethylaminomethylphenol.

[0052] The concentration of catalyst is preferably at least 0.05 phr andmore preferably at least 0.1 phr. It is preferably less than 3 phr andmore preferably no more than 1 phr. (For the purposes of thisapplication only, “phr” or “parts per 100 parts resin” refers to theparts of a material per 100 parts of the combined low-viscosity epoxyresin and chain extender, by weight.)

[0053] The present invention also contains an inhibitor which inhibitsthe activity of the catalyst during B-staging. The inhibitor is a Lewisacid. Examples of preferred inhibitors include halides, oxides,hydroxides and alkoxides of zinc, tin, titanium, cobalt, manganese,iron, silicon, boron, aluminum and similar compounds (other than boronhalides)—for instance boric acid, boroxines (such astrimethoxyboroxine), boron oxide, alkyl borates, zinc halides (such aszinc chloride) and other Lewis acids that tend to have a relatively weakconjugate base. When the formulation is intended for use in electricallaminates, then the inhibitor preferably contains no significant levelsof halide. The most preferred inhibitor is boric acid. Boric acid asused herein refers to boric acid or derivatives thereof, includingmetaboric acid and boric anhydride. The formulation preferably containsat least 0.3 moles of inhibitor per mole of catalyst, and morepreferably contains at least 0.6 moles of inhibitor per mole ofcatalyst. The formulation preferably contains no more than 3 moles ofinhibitor per mole of catalyst and more preferably contains no more than2 moles of inhibitor per mole of catalyst.

[0054] The inhibitor and catalysts may be separately added to thecompositions of this invention, or may be added as a complex. Thecomplex is formed by contacting and intimately mixing a solution of theinhibitor with a solution of the catalyst. Optionally, an acid having aweak nucleophilic anion may be present. Such contacting generally isperformed at ambient temperature, although other temperatures may beused, for example, temperatures of from 0° C. to 100° C., morepreferably from 20° C. to 60° C. The time of contacting is thatsufficient to complete formation of the complex, and depends on thetemperature used, with from 1 to 120 minutes preferred, and 10 to 60minutes more preferred.

[0055] It is theorized, without intending to be bound, that theadvancement reaction of epoxy resin with phenolic chain extender and thecuring reaction of epoxy resin with epoxy resin usually occurssimultaneously. If catalyst is added to increase one reaction duringB-staging, the rate of the other reaction is also increased. On theother hand, inhibitors used in the present invention retard the curingreaction of epoxy resin with epoxy resin at B-stage temperatures, buthave no effect on or accelerate the advancement reaction of epoxy resinwith phenolic chain extender in the B-stage. The effect of the inhibitoris reduced at higher temperatures, so that epoxy-epoxy reactions maytake place in the lamination step. This makes it easy to control thegrowth of viscosity in the B-stage by selection and proportions oflow-viscosity epoxy resin and phenolic chain extender.

[0056] The formulation preferably further contains a low proportion oforganic solvent. It more preferably contains a mixture of solventsuseful for dissolving the individual components in the formulation. Thepreferred volatile organic solvents to dissolve epoxy resins are wellknown, and it is known how to use them to obtain desired viscosity.Examples of suitable solvents are taught in H. Lee & K. Neville,Handbook of Epoxy Resins at 24-31 (McGraw-Hill Book Co. 1967). Examplesof preferred solvents include ketones (such as methyl ethyl ketone,methoxyacetone or acetone); ethers; esters; glycols; glycol ethers;C₁-C₈ alcohols; and aromatic hydrocarbons (such as xylenes). Thecatalyst and the inhibitor are preferably dissolved in polar solvents,such as dimethylsulfoxide (DMSO), g lycerine and dimethylformamide, withalcohols having from 1 to 6 carbon atoms and glycols having from 2 to 6carbon atoms being more preferred. The solvent may also contain up to 30percent water as disclosed in European Patent Application 0 567 248.

[0057] The formulation preferably contains no more than 20 percentvolatile organic solvent and more preferably no more than 15 percent byweight. The formulation may contain as little as 0 percent volatileorganic solvent but preferably contains at least 5 percent volatileorganic solvent by weight.

[0058] The formulation preferably further contains a multifunctionalcross-linker. Such multifunctional cross-linkers are described innumerous references, such as Vol. 6, Encyclopedia of Poly. Sci. & Eng.,“Epoxy Resins,”at 348-56 (J. Wiley & Sons 1986). Examples of suitablemultifunctional cross-linkers include known curing agents for epoxyresins, such as polyamines, polyamides, polyanhydrides, polyphenols andpolyacids that contain more than two reactive sites per molecule onaverage. Preferred examples of multifunctional cross-linkers includedicyandiamide and polyphenols such as novolacs. Examples of othermultifunctional cross-linkers which can be used include polyanhydridesthat are taught in PCT Publication WO94/11415 (published May, 26, 1994).

[0059] Multifunctional cross-linkers (as opposed to catalysts and chainextenders) preferably contain on average more than two active hydrogenmoieties per molecule. For instance, the cross-linker preferablycontains a plurality of secondary amine groups, one or more primaryamine groups, more than 2 phenolic hydroxyl groups, a plurality ofprimary amide groups or more than two carboxylic acid groups.

[0060] The quantity of multifunctional cross-linker is preferablyselected such that the formulation contains a stoichiometric excess oflow-viscosity epoxy resin over the combination of phenolic chainextender and multifunctional cross-linker. (For the purposes of thisapplication, dicyandiamide is taken as having 5 to 7 curing sites permolecule.) The formulation preferably contains no more than 0.75equivalents of chain extender and cross-linker per epoxide equivalent,more preferably no more than 0.6 equivalents, and most preferably nomore than 0.5 equivalents. The formulation may contain 0 equivalents ofmultifunctional cross-linker, but preferably contains at least 0.01equivalents per epoxide equivalent, more preferably contains at leastabout 0.05 equivalents, and most preferably contains at least 0.1equivalents. When the multifunctional cross-linker is dicyandiamide, theformulation preferably contains at least 0.05 phr and more preferably atleast 0.1 phr of dicyandiamide. It preferably contains no more than 2.25phr and more preferably no more than 1.5 phr.

[0061] The formulation preferably further contains a stabilizer toprevent premature reaction of the low-viscosity epoxy resin and thechain extender during storage and shipping. Strong inorganic and organicacids and the anhydrides and esters of said acids (including half estersand part esters) have been found to be particularly effective asreaction inhibitors. By the term “strong acid” it is meant an organicacid having a pK_(a) value below 4, preferably below 2.5. Examples ofpreferred reaction inhibitors include inorganic acids such ashydrochloric acid, sulfuric acid and phosphoric acid; inorganic acidanhydrides such as phosphoric acid anhydride (P₂O₅); esters of inorganicacids such as alkyl, aryl and aralkyl and substituted alkyl, aryl andaralkyl sulfonic acids such as p-toiuene sulfonic acid and phenylsulfonic acid and stronger organic carboxylic acids such astrichloroacetic acid and alkyl esters of said acids, such as the alkylesters of p-toluene sulfonic acid, for example, methyl-p-toluenesulfonate, and ethyl-p-toluene sulfonate and methanesulfonic acidmethylester. More preferred reaction inhibitors include alkyl esters ofsulfuric acid, arylsulfonic acids or aralkylsulfonic acids. Mostpreferably, the stabilizer is a lower (C₁-C₆) alkyl ester of p-toluenesulfonic acid. The quantity of stabilizer is preferably 0 to 1 phr.

[0062] The viscosity of the formulation at 20° C. is preferably no morethan 800 mPa·s, and more preferably no more than 500 mPa·s. (Viscosityis measured by a CANNON-FENSKE viscometer according to the ordinaryinstructions for operation.) The viscosity of the formulation ispreferably at least 50 mPa·s and more preferably at least 100 mPa·s.

[0063] The formulations previously described may be used to makeelectrical laminates as described in the background of the invention.The formulations have relatively low-viscosity, but advance quickly andcontrollably during B-staging to avoid drip, and cure to provide goodlaminates in the laminating step. Moreover, the presence of inhibitor inthe formulation increases the total heat of reaction released by curingthe formulation. This suggests that the formulations have increasedcross-link density when cured.

[0064] For packaging and shipment, it may be convenient to supply apreliminary formulation as described in the second aspect of theinvention. The preliminary formulation contains low-viscosity epoxyresin, phenolic chain extender, inhibitor, and a solvent as previouslydescribed. The ratios are also as previously described, except that theformulation preferably contains no more than 15 weight percent solvent,and more preferably no more than 10 weight percent solvent. Theformulation preferably contains at least 1 percent solvent and morepreferably at least 5 percent solvent. The formulation preferablycontains at least 0.05 phr inhibitor, and more preferably at least 0.2phr inhibitor. It preferably contains no more than 2 phr inhibitor, andmore preferably no more than 1 phr inhibitor.

[0065] The preliminary formulation preferably contains a less thancatalytic quantity of curing catalyst. For instance, the concentrationof catalyst preferably contains less than 0.1 phr catalyst, morepreferably less than 0.05 phr catalyst, and most preferably less than0.01 phr catalyst. The preliminary formulation also preferably containsless cross-linker than would be required to cure the formulation. Theconcentration of cross-linker in the preliminary formulation ispreferably less than 0.1 equivalents per epoxide equivalent, morepreferably less than 0.05 equivalents, and most preferably less than0.01 equivalents. The concentrations of catalyst and curing agent aremost preferably about 0.

[0066] Such formulations can be stored for long periods without loss ofstability. The invention is illustrated with greater specificity in thefollowing specific examples.

EXAMPLE 1 Formulations Containing Boric Acid Inhibitor and DicyandiamideCross-linker

[0067] Six formulations were made. Each formulation contained 65.5 partsD.E.R.* 330 (*Trademark of The Dow Chemical Company) liquid epoxy resin,34.5 parts tetrabromobisphenol A and 5.26 parts methyl ethyl ketone. A10 percent solution of boric acid in ethanol was added to the blend toprovide the quantity of boric acid shown in Table I.

[0068] After the formulations were aged overnight, the followingadditional components were added: 9.33 parts of a solution containing7.5 parts dicyandiamide dissolved in 30.5 parts dimethylformamide and 62parts propylene glycol monomethyl ether; 1.25 parts of a 20 percentsolution of 2-methylimidazole in ethanol; and sufficient methyl ethylketone to dilute the formulation to 80 percent solids. The formulationswere loaded into an aluminum calorimetry capsule. Each formulation wasplaced under vacuum at room temperature for a period of one hour to drawoff the solvent. The advancement and curing of each formulation TABLE IBoric Acid Formulation (parts) A* 0 1 0.1 2 0.2 3 0.3 4 0.4 5 0.5

[0069] was studied by differential scanning calorimetry (DSC) using aMETTLER DSC30 calorimeter and a temperature increase of 10° C. perminute.

[0070] In Formulation A, a broad advancement peak ran from about 105° C.to a maximum of 160° C., and a sharp curing peak hit a maximum at 165°C., essentially on top of the advancement peak. In Formulation 1, theadvancement peak remained at a maximum of about 160° C., but the maximumof the curing peak had moved slightly to about 170° C. In Formulation 2,the two peaks were almost completely separated, as advancement showed abroad peak at 160° C. and curing showed a broad peak at about 195° C. InFormulations 3,4 and 5 the peaks for advancement and curing wereessentially separate. The advancement peaks ran from about 110° C. toabout 170° C., with a maximum at about 15.5. The curing peak ran fromabout 175° C. to about 225° C. with a maximum at about 205° C. Theexperiments show that when the concentration of boric acid in theformulation increases, the advancement reaction and the curing reactionseparate into two main different peaks which occur at two differenttemperatures. This separation allows rapid, controllable advancement inthe B-stage without excessive curing.

EXAMPLE 2 Formulation Containing Boric Acid and Phenolic Cross-linker

[0071] A formulation was made that contained: 65.5 parts D.E.R.* 330(*Trademark of The Dow Chemical Company) liquid epoxy resin, 34.5 partstetrabromobisphenol A, 5.26 parts methyl ethyl ketone, and 5 parts of asolution containing 20 weight percent boric acid in methanol. Thesolution was aged overnight. Then the following components were added: 4parts of a solution containing 50 weight percent PERSTORP 85-36-28phenol novolac resin (commercially available from PERSTORP AS, Perstorp,Sweden and containing 4 to 5.5 phenolic hydroxyl groups per molecule) inacetone; 3.5 parts of a solution containing 20 weight percent2-methylimidazole dissolved in methanol; and sufficient methyl ethylketone to dilute the formulation to 80 percent solids. The advancementand curing of the formulation was studied by DSC as described inExample 1. The scan exhibits a broad advancement peak from 100 to 160with two maxima at about 135° C. and 145° C., and a broad curing peakfrom about 160° C. to 220° C. with a maximum height at about 210° C. Theexperiment shows that a similar separation of advancement and curing areobserved when the cross-linker is a phenolic resin, rather thandicyandiamide.

EXAMPLE 3 Preparation of Prepregs

[0072] A formulation was prepared as described in Example 2, except thatthe cross-linking agent contained a 50/50 mixture of the PERSTORP phenolnovolac and tetraphenolethane. The viscosity of the formulation wasmeasured at 25° C. using an ICI cone and plate set using Cone “C”, andwas 80 mPa s. The formulation was applied to a substrate of glass cloth(Type 7628 which can be purchased from Porcher Textile, Badinieres,Fr-38300 Bourgoin-Jallieu France, or Interglas Textil GmbH, Ulm/Donau,Germany) by dipping. The impregnated substrates were passed through aCARATSCH pilot treater (built by Caratsch AG, Bremgarten, Switzerland)having a 3 meter horizontal oven at a temperature of 152° C. and atspeeds of 0.8, 1.0, 1.2 and 1.8 m/minutes to B-stage the resin and forma prepreg. The prepregs were cracked, and powders of the B-staged resinswere recovered. The B-staged resin was cured in a differential scanningcalorimeter according to the procedure in Example 1. The first threeexperiments provided essentially identical results; a single broad peakfrom about 150° C. to 280° C., with a maximum height at about 210° C.The final experiment, at 1.8 m/min, showed the same broad peak, but alsoshowed a slight broad hump from about 100° C. to 150° C. with a maximumat about 135° C. We attribute the hump to uncompleted advancement. Theexperiments show that, within reason, the formulation provides arelatively controlled and consistent B-staging, regardless of thetreater speed and residence time.

EXAMPLE 4 Preparation of Laminates

[0073] Formulations are prepared as described in Examples 1 and 2 andshown in Table II. Each formulation contained 80 weight percent solidsand 20 weight percent volatile organic solvents. The formulations wereapplied to glass cloth as described in Example 3. The glass cloths werepassed through a CARATSCH pilot treater having a 3 meter horizontal ovenat an air temperature as shown in Table II, using a winding speed shownin Table II. The resin content of each prepreg was measured using 10cm×10 cm square sheets of glass cloth before and after prepregproduction, according to Method IPC-L-109B, IPC-TM-650: 2.3.16(available from the Institute for Interconnecting and PackagingElectronic Circuits, Lincolnwood, Ill., USA.) Results appear in TableII. Eight sheets of each prepreg were laid-up in alternating layers withsheets of copper foil. The laid-up prepregs were cured under a pressureof 250 N/cm² according to the following temperature profile: temperaturewas raised from room temperature to 170° C. over a period of 40 minutes,then maintained at 170° C. to 185° C. for a period of 60 minutes, thencooled from 185° C. to room temperature over a period of 20 minutes. Theresulting laminates received no post-treatment.

[0074] The following tests were performed on each cured laminate:

[0075] (a) N-methylpyrrolidone (NMP) pick-up was measured by weighing a5 cm×5 cm sheet of laminate, immersing it in NMP at 23° C. for 30minutes, and then reweighing. The results are shown in Table II,expressed as a percent gain.

[0076] (b) Copper peel strength was measured by Method IPB-L-115B,IPC-TM-650: 2.4.8. The results are shown in Table II expressed in N/cm.

[0077] (c) Laminate glass-transition temperature was measured using theDSC from Example 1, scanning from 50° C. to 220° C. at 10° C. per minuteand taking the second scan as the glass-transition temperature. Theresults are expressed in Table II in ° C.

[0078] (d) Water resistance was measured by putting the laminates in apressure cooker for 120 minutes according to Method IPC-A-600,IPC-MI-660 and IPC-TM-650:2.6.16. All laminates passed the test with 100percent. TABLE II 11 12 Formulation Liquid Epoxy Resin/TBBA blend (partssolid) 100 100 Boric acid (phr solid) 1.0 0.3 Phenolic cross-linker (phrsolid) 2.0 — Dicyandiamide (phr solid) — 0.7 2-methylimidazole (phrsolid) 0.7 0.5 Varnish % solids (wt %) 80 80 Varnish viscosity(mPa.s)⁽¹⁾ 80 110 B-staging Treater setting 2 2 Treater Temperature (°C.) 152 170 Treater Speed (m/min) 1.0 0.9 Prepreg resin content(percent) 43 45 Laminate properties NMP-pick-up (wt %) 0.05 0.08 Copperpeel strength (N/cm) 15.1 17.3 Laminate Tg (° C.) 146 138

What is claimed is:
 1. A formulation comprising: (1) a low-viscosityepoxy resin; (2) a phenolic chain extender whose concentration is from0.1 to less than 0.6 equivalents of phenolic hydroxyl group pereuivalent of the low-viscosity epoxy resin; (3) a catalyst that promotesself-curing reactions between epoxy groups; (4) an inhibitor which is aLewis acid in an inhibiting amount of from 0.1 phr to 2 phr. (5) lessthan 25 weight percent of a volatile organic solvent; and (6)optionally, a multifunctional cross-linking agent.
 2. A preliminaryformulation comprising: (1) a low-viscosity epoxy resin; (2) a phenolicchain extender whose concentration is from 0.1 to less than 0.6equivalents of phenolic hydroxyl group per equivalent of thelow-viscosity epoxy resin; (3) a Lewis acid in an inhibiting amount offrom 0.1 phr to 2 phr; and (4) no more than 20 weight percent of avolatile organic solvent.
 3. A formulaton as described in any of thepreceding claims wherein the low-viscosity epoxy resin is a liquiddiglycidyl ether of a dihydric phenol.
 4. A formulation as described inany of the preceding claims wherein the chain extender is a dihydricphenol or a halogenated variation thereof having a melting point higherthan 100° C.
 5. A formulation as described in any of the precedingclaims wherein the inhibitor is a halide, oxide, hydroxide or alkoxideof zinc, tin, titanium, cobalt, manganese, iron, silicon, boron oraluminum, other than a boron halide.
 6. A formulation as described inany of the preceding claims wherein the inhibitor is boric acid,metaboric acid, boroxine, boron oxide or alkyl borates.
 7. A formulationas described in any of claims 1 or 3 to 6 wherein the multifunctionalcross-linking agent is a polyamine, polyamide, polyanhydride,polyphenol, or polyacid compound that contains more than two activesites per molecule on average, and the concentration of multifunctionalcross-linker is at least 0.05 equivalents per epoxide equivalent.
 8. Aformulation as described in any of claims 1 or 3 to 7 wherein theformulation contains from 0.2 to 0.5 equivalents of chain extender(based on phenolic hydroxyl groups) per epoxy equivalent, at least 0.01equivalents of cross-linker per epoxy equivalent and no more than 0.75combined equivalents of chain extender and cross-linker per epoxyequivalent.
 9. A formulation as described in any of claims 1 or 3 to 8wherein the catalyst contains amine, phosphine, heterocyclic nitrogen,ammonium, phosphonium, arsonium or sulfonium moieties, and has aconcentration of from 0.05 to less than 3 parts per 100 parts resin, byweight.
 10. A formulation as described in any of claims 1 or 3 to 9 thatcontains less than 20 percent volatile organic solvent.
 11. Use of aformulation as described in any of claims 1 or 3 to 10 in a process tomake electrical laminates, said process containing the steps of: (1)applying the formulation to a substrate by rolling, dipping, spraying,other known techniques and/or combinations thereof; (2) heating theimpregnated substrate as a temperature sufficient to draw off solvent inthe epoxy formulation and optionally to partially cure the epoxyformulation, whereby a prepreg is formed; (3) stacking one or moresheets of prepreg in alternating layers with one or more sheets of aconductive material; and (4) pressing the laid-up sheets at hightemperature and pressure for a time sufficient to cure the resin andform a laminate.
 12. A formulation as described in any of claims 2 to 8wherein the formulation contains less than 0.05 phr curing catalyst andless than 0.05 equivalents of multifunctional cross-linker per epoxideequivalent.